Now that the first shoe has dropped in what should be a busy trade season for the Celtics, with Courtney Lee sent off to Memphis for Jerryd Bayless' expiring contract, the first question is who's next? But another, and just as important, question should be who should stay?

When it comes to who is untouchable, the short answer is no one. Anyone can be had for the right price. This will be more of a look at who can be a legit building block going forward.

Avery Bradley

Bradley will be a restricted free agent at the end of this season, which might make him expendable if Boston doesn't think they can retain his services. He reportedly turned down the Celtics offer of four years for $24 million seeking a deal that will pay him at least $8 million per season. Whether or not he's worth that kind of money is a topic for another day. What matters is if another team will give him that money or if the Celtics can meet somewhere in the middle to keep their defense-first guard.

History tells you that restricted free agency isn't that scary. Other teams usually don't make offers on players knowing their current team will match (ask Atlanta's Jeff Teague) and Boston should be able to retain Bradley if they want to, and they should.

On the court, Bradley has taken meaningful steps in his offensive game, while remaining arguably the best one-on-one defender in the NBA. He's averaging a career-high 14.3 ppg and increased his field goal percentage from 40.2 percent last year to 45.4 percent this year. He's also averaging just over one made three per game. As the season goes on, Bradley seems to be getting better offensively, as his points per game have increased each month to the point where he is averaging 17.2 ppg in five games this month.

While those numbers aren't eye popping and the reasons Bradley has had a bigger role in the offense are because of the void left by the Kevin Garnett-Paul Pierce trade and no Rajon Rondo yet, the improvement shouldn't be discounted. Yes, Bradley had to step up, but the fact that he's actually doing it is a big deal (right, Jeff Green?).

Two-way players are very hard to come by in this league and if Bradley keeps improving that jumper, he could be a monster. Remember, Bradley is still only 23 years old and if he keeps the trend line going upward he could be a great option on a contender. Hopefully that contender will be Boston.

Jared Sullinger

An argument could be made that Sullinger has been the best player on the Celtics this year. His rebounding is exceptional and he doesn't look to have suffered any long-term effects from off-season back surgery. Besides being a big body to clog the paint, Sullinger has started to bring out that sweet shooting touch that he showed off during the Summer League in his rookie year.

The biggest issue lately with Sullinger has been his maturity. Even Sullinger's biggest fans have to raise an eyebrow to him receiving three flagrant fouls in the past two games (one was later rescinded). Sullinger was also arrested for a domestic incident with his girlfriend this past offseason so questions about his temperamant are more than fair.

If he can reel that emotion in and channel it into being a productive leader on the court, Sullinger could be the glue guy on this team. He could pick up where Garnett left off in being the heart and soul of the franchise. He's got the talent and the mindset to be a leader, those are qualities you build around.

Rajon Rondo

Can a team win the title with Rondo as their best player? No. Can they with Rondo as the second best player? Absolutely. That's not something to be taken lightly. I've gone on record multiple times saying the Celtics should trade Rondo, but the way this team is playing with this coach has me rethinking that idea. The biggest caveat to holding onto Rondo has to be the Celtics landing one of the coveted Top 5 picks in the upcoming draft. This team desperately needs a center and a wing. It just so happens that there is one amazing center (Joel Embiid) and a couple amazing wings (Jabari Parker and Andrew Wiggins) projected to go in the Top 5.

Could you imagine a team composed of Embiid-Sullinger-(player x)-Bradley-Rondo going forward? And Boston GM Danny Ainge has been stockpiling assets so whichever position the Celtics didn't draft they could acquire by trading away one or two of their future first round picks. That's a team that is not too far away from contending again.

If not, if the Celtics start winning games again and find themselves picking at the end of the Top 10 or in the teens, then Rondo has to be dealt for a young blue-chipper on the cusp of becoming a franchise guy. As nice as the core of Sullinger-Bradley-Rondo is, it's missing that #1 piece. If the Celtics can't get that piece in this year's draft, they have to deal their best asset to get it.